1. Field of the Invention
This invention pertains to the general field of testing of semiconductor materials. In particular, it pertains to an improvement in the probe used for characterizing the properties of electroluminescent semiconductor wafers.
2. Description of the Prior Art
The characterization of semiconductor materials and in particular light-emitting semiconductor structures at the wafer-level (i.e., after forming the p-n junction and the active quantum well layers, but prior to the chip processing steps) is typically carried out with a non-destructive wafer probe. A conductive probe is temporarily placed in contact with the top of the epi-wafer (p-GaN) layer while an electrode contacts the n-GaN layer through either the edge of the wafer or through other means that allow access to the n-GaN layer. Such typical layout is illustrated in FIG. 1. When energized, the conductive probe, the semiconductor p-n junction structure on the wafer and the electrode form a temporary light-emitting device. By injecting a known current into the junction, light will emit from the device and the spectral properties and their relationship with the electrical properties can be measured and characterized.
Although the method of using conductive probes for semiconductor wafer measurements and tests has been known in the field, the issues of making good, consistent probe-wafer contact with repetitive results are still problematic challenges that vary from application to application. For light-emitting wafer testing, a well-defined uniform contact area with minimal contact resistance is essential. Therefore, the probe material should be stable under a variety of electrical drive conditions.
One major challenge is the precise estimation and consistent repetitiveness of the true contact area between the probe and the surface of the wafer, which affects conductivity and all related measurement parameters. A hard metallic probe would be ideal for perfectly flat and smooth surfaces because of the high and uniform conductivity of metals. However, the surface of wafers is typically not perfectly smooth, but it contains a degree of roughness sufficient to create non-uniformities in the way the probe contacts the wafer. Therefore, different probes with softer tips have been used to cause the probe to deform under pressure and conform to the profile of the wafer's surface. For example, U.S. Pat. No. 7,679,381 (issued to Ma) describes a probe that includes a conductive deformable tip made of elastomer or polymer material and a pressure control that together ensure a good contact with the wafer under test as various measurements are taken across its surface.
Another type of probe used to improve the uniformity of contact over the test wafer consists of a traditional metallic probe with a conductive silicone tip, such as RTV (Room Temperature Vulcanization) liquid silicone material. Such conductive silicone consists of metallic flakes, typically silver particles, with a nominal diameter in the order of micrometers, dispersed in a silicone carrier that is attached and cured onto the tip of the metal probe. (Depending on the application, other conductive fillers may be used, such as graphite, silver-coated copper, nickel, and so on.) In use, the probe is pressed onto the surface of the wafer, causing its deformable tip to conform to wafer surface irregularities, thereby providing a substantially uniform contact throughout.
FIG. 2 illustrates a probe station for light-emitting epi-wafer characterization using a conducting silicone probe of the type described above, or any other deformable probe, including the novel probe of the invention described below. A spring-loaded probe 10 is used as an anode electrode. The probe is engaged to the surface S of the wafer W and makes contact with the p-GaN layer. The loading force applied to the probe for contacting the wafer is controlled by a spring-loaded mechanism acting on the probe 10 and by controlling the distance between the probe mount and the wafer surface. The contact of the cathode 12 is made at the side edge of the wafer so that the n-GaN layer can be accessed. The two electrodes 10,12 together with the epi-wafer form a temporary LED structure. When current is injected into the wafer from the probe 10, luminescence under the probe occurs and the emitted light is collected from sensors both in the front and back sides of the wafer through optical fibers 14 and a spectrometer 16. The data output of the spectrometer is acquired by a computer processor 18 for analysis and display. A power source 20 controls the level of current injected into the wafer W. The measurements are made on the sample surface by moving the probe or the sample sequentially between locations of interest. After one or a series of measurements have been taken at a location, a stage (not shown) moves the sample or the probe to the next sample location for the next measurement.
While the conductive silicone tip used on a spring-loaded probe as described above produces significant improvements over conventional metallic and other soft-tip probes, certain problems remain unsolved. For instance, when the conductive silicone tip is pressed against the wafer surface, even a very smooth surface, each metal flake incorporated in the silicone tends to make a point contact with the wafer and only silicone material contacts the areas surrounding the points of contact. Thus, while the quality of the contact is uniform in a coarse sense, it is very non-uniform in a microscopic sense and, the conductivity of silicone being negligible in comparison to that of metal, hot spots tend to form in point-contact areas. The present invention describes a new structure and a method of manufacture for a probe that embodies both the conformability and the uniform conductivity that are essential for reliable and repeatable light-emitting semiconductor wafer testing.